PDM performance simulation and testing

ABSTRACT

A method for measuring load performance of a positive displacement motor (PDM) test coupon. The test coupon comprises a partial length of a PDM stage and is received inside a sealable test chamber. In some embodiments, the test coupon may be cut from a failed PDM stage. The test chamber is filled with test fluid. In some embodiments, the test fluid may be drilling fluid sampled from a live well. Rotation of the rotor on the test coupon actuates rotation of the stator. A braking torque is applied to the stator rotation, enabling evaluation of, for example, fatigue load performance of test coupon. Additional embodiments comprise the rotor axis and the stator axis being offset in order to simulate rotor/stator eccentricity in a full size PDM stage.

RELATED APPLICATIONS

This application is a continuation of, commonly-invented andcommonly-assigned U.S. patent application Ser. No. 15/464,640 filed Mar.21, 2017 (now U.S. Pat. No. 9,938,929), which in turn claims the benefitof, and priority to, commonly-invented and commonly-assigned U.S.Provisional Patent Application Ser. No. 62/311,278 filed Mar. 21, 2016.The entire disclosure of 62/311,278 is incorporated herein by reference.

FIELD OF THE DISCLOSURE

This disclosure is directed generally to devices that test theperformance of a subterranean positive displacement motor (“PDM”), andmore particularly to a device that miniaturizes the performance testingof full-scale PDMs while still maintaining accurate simulation ofdownhole conditions during such miniaturized performance testing.

BACKGROUND OF THE DISCLOSED TECHNOLOGY

Currently, PDMs are tested using standardized ASTM protocols in order tosimulate performance of parameters such as elastomer materialperformance and elastomer bonding performance under projected drillingloads. Conventional testing may be performed on failed PDM parts(typically stators) that have been retrieved from downhole service aspart of an investigation into the cause of the failure. Alternatively,conventional testing may be performed as part of well planning activity,prior to drilling, in order to optimize selection of PDM components suchas stator elastomer in view of the expected downhole environmentalconditions and anticipated loading.

Conventional testing is done according to current applicable ASTMprotocols. The ASTM tests are not particularly representative ofspecific expected or encountered downhole environments. First, the ASTMtests are not able to replicate the cyclic loading stresses encounteredby PDMs in service. Second, the ASTM tests only peripherally simulatethe elevated temperatures and pressures encountered by PDMs in service.

Generally, the ASTM tests are basic immersion tests in which anelastomer sample from a stator is first exposed to drilling fluid for aprescribed period at elevated temperatures, followed by performanceevaluation by pull or tear testing. The performance evaluation isnormally done at ambient temperatures and pressures. The purpose of theASTM testing is to evaluate loss of physical properties in the rubbermaterial and/or bond integrity. The ASTM testing may be done with asmall volume of drilling fluid used in the specific drillingapplication. The drilling fluid sample may be taken from the actualdrilling site.

Examples of specific conventional ASTM protocols include the following:

ASTM D2240—Durometer Hardness (Shore A)

ASTM D412—Tensile properties (Die C)

ASTM D5289—Vulcanization using Rotorless Cure Meter (MDR tests)

ASTM D624—Tear Strength (Die C)

ASTM D3182—Practice for preparing standard vulcanized sheets (fortensile and tear bars)

ASTM D429—Adhesion

ASTM D6370—Compositional Analysis by Thermogravimetry (TGA)

ASTM D471—Rubber Property—Effect of Liquids (Swell or immersion test)

In addition to the traditional ASTM testing, two other conventional testmethods are known to evaluate the performance of PDM power sections. Incontrast to the traditional ASTM protocols described above, however,these two test methods require large volumes of test drilling fluid toperform each test.

Flow loop testing attempts to simulate downhole PDM service by placing afull downhole motor in an enclosed dynamometer (“dyno”). Drilling fluidis introduced at full operating pressure to drive the PDM power section.It often takes hundreds or even thousands of gallons of drilling fluidto fill and operate the flow loop. Each well drilled may have differentdrilling fluid types, making a full scale dyno test cost prohibitive toperform on a frequent basis. Further, the specialized equipment requiredto handle drilling fluids under pressure and high temperature make thisapproach logistically challenging and often not economically viable.

Recirculating pump vessels have also been used in the past to evaluatethe performance of PDMs. While pump vessels do not require the volume ofdrilling fluid needed by flow loops, pump vessels still require aminimum of 25 gallons of drilling fluid to operate. Further, pumpvessels have limited load ranges that correspondingly limit the range ofdownhole environments that can be simulated.

The volumes of drilling fluid needed by flow loop testing andrecirculating pump vessel testing present a further drawback that isinherent to these conventional tests. Often the need for testing andevaluation arises after there has been a specific downhole PDM failure.Investigation of the failure advantageously includes evaluation andtesting of the power section in an environment that closely simulatesthe downhole conditions in which the failure occurred. Such simulationis enhanced greatly when the evaluation and testing can be done using asample of the actual drilling fluid flowing through the power section atthe time the failure occurred. Such samples can be taken, and areavailable, but they come in small volumes, typically 0.5 to 2gallons—not nearly enough to do flow loop or recirculating pump vesseltesting.

There is therefore a need in the art for a test apparatus that canclosely simulate, evaluate and measure PDM performance under (1) theelevated temperatures encountered downhole, and (2) the cyclic loadsencountered downhole. The test apparatus should nonetheless provideembodiments in which only small quantities of drilling fluid arerequired, ideally less than about 10 gallons of drilling fluid,preferably less than about 5 gallons and most preferably, less thanabout 2 gallons. Embodiments of the test apparatus should also ideallyhave a small enough footprint to be suitable to be housed in aconventional laboratory or at a wellsite location.

SUMMARY AND TECHNICAL ADVANTAGES

These and other drawbacks in the prior art are addressed by a testapparatus using a PDM test coupon that is a partial length of one stageof power section, mounted within a test chamber. In some embodiments,the longitudinal length of the test coupon is up to about 12 inches. Thetest chamber also holds a small volume of test fluid, sufficient toimmerse the test coupon and occupy all of its internal progressing gaps.In some embodiments, the test fluid is drilling fluid maintained atpredetermined and monitored elevated temperatures so as to simulatetemperature performance in desired downhole conditions. The volume oftest fluid/drilling fluid required to fill the test apparatus is furtheradvantageously small enough to allow, if desired, use of actual drillingfluid samples drawn from full operational power sections that may havefailed downhole. It will be appreciated that the test coupon cannot besealed because it is only a partial length of one stage of a PDM powersection, and thus presents less than one complete progressing cavity.Therefore, the test apparatus does not pump drilling fluid through thetest coupon per the conventional operation of a PDM. Instead, anexternal motor rotates the rotor section in the test coupon, actuatingcorresponding rotation of the stator section by contact between rotorsection and the stator section. The rotation of the rotor section, withcorresponding actuated rotation of the stator section, is all in thepresence of test fluid in the progressing gap(s) between rotor sectionand stator section in the test coupon. In this way, traditional PDMinteraction between rotor and stator via drilling fluid flow can besimulated without pumping drilling fluid. The test fluid in the testcoupon is present to interact with the test coupon materials to simulatedegradation that might be seen in actual downhole conditions. Note thatthe relative differential rotation of the rotor section and the statorsection in the test coupon may cause the test fluid to flow under lowpressure through progressing gaps between rotor and stator sections. Insome embodiments, a flow loop of test fluid may form through two or moreprogressing gaps. In other embodiments where openings are provided inthe stator section, a flow loop of test fluid may form through theprogressing gaps, out of the stator section openings, into the annularspace between the test coupon and the test chamber, and back into theprogressing gaps again.

Once the rotor section and stator section are rotating, an externalbrake mechanism intentionally applies a braking torque to the rotationof the stator section against the externally-driven rotation of therotor, causing the test coupon to come under controlled load. Thebraking force may be controlled, for example, by measuring the outputtorque of the stator section and adjusting the braking force to obtain adesired torque. This ensures the interface stress between the rotor andstator sections remains constant, resulting in a highly controlledfatigue loading being placed on the test coupon. Controlling the appliedtorque in this way enables multiple modes of evaluating performance ofthe test coupon. The performance of the test coupon in the test fluidenvironment can be monitored continuously over a wide range of loadsplaced cyclically on the coupon over time. In this way, a stress fieldcan be generated between the rotor and stator sections in the testcoupon that simulates very closely the same loading placed on acorresponding full scale power section in downhole service. Variablessuch as materials selection, performance optimization and usefuldownhole life, for example, can all be evaluated against the fatiguecaused by the loading placed on the test coupon. Performance of the testcoupon may be evaluated via measuring metrics including, but not limitedto: (1) recording cycles to test coupon failure, via, for example,counting rotor section and/or stator section cycles through to failure,(2) examining the rotor section and/or the stator section for wear, (3)monitoring temperature change over time in the test chamber, and (4)monitoring temperature change over time in an elastomer layer providedby the stator section at its contact surface with the rotor section.

In other embodiments, an external motor may drive the stator section inthe test coupon, and an external brake mechanism may intentionally slowthe rotor section in order to place load on the test coupon.

In other embodiments, the separate external motors may drive the rotorsection and the stator section in the test coupon. Controlleddifferential rotation of separate motors driving the rotor and statormay be used to place the test coupon under desired loads.

In other embodiments, the disclosed test apparatus provides a method ofevaluating PDM power section materials at elevated drilling temperaturesand under similar stress profiles as seen in downhole service by fullscale PDM power sections. According to currently preferred embodiments,the test apparatus design uses a small section of the full scale PDMpower section as a test coupon and preferably less than about twogallons of drilling fluid within the test chamber. Many of theadvantages of conventional flow loop or recirculating pump vesseltesting can thus be obtained using the disclosed test apparatus, whileat the same time obviating the need for large footprint or high volumesof drilling fluid.

According to a first aspect, therefore, this disclosure describes anembodiment of a method for measuring load performance of a positivedisplacement motor (PDM) test coupon, the method comprising the stepsof: (a) providing a PDM test coupon, the test coupon comprising apartial length of a PDM stage, the test coupon including a rotor sectionreceived inside a stator section, wherein the rotor section and thestator section are independently rotatable, wherein further at least oneprogressing gap on a helical pathway is formed between the rotor sectionand the stator section when the rotor section and the stator section aredifferentially rotated; (b) receiving the test coupon inside a sealabletest chamber; (c) filling the test chamber with test fluid; (d) sealingthe test chamber; (e) rotating the rotor section, thereby actuatingcorresponding rotation of the stator section in the presence of testfluid in the progressing gap; (f) applying a braking torque to saidstator section rotation actuated in step (e); and (g) responsive to step(f), evaluating performance of the test coupon, wherein said evaluatingstep includes at least one substep selected from the group consistingof: (g1) controlling torque across the rotor section and the statorsection; (g2) counting, through to failure of the test coupon, at leastone of (1) rotor section rotation cycles and (2) stator section rotationcycles; (g3) examining at least one of (1) the rotor section and (2) thestator section for wear; and (g4) monitoring temperature change overtime in the test chamber.

According to a second aspect, this disclosure describes an embodiment ofa method for measuring load performance of a positive displacement motor(PDM) test coupon, the method comprising the steps of: (a) providing aPDM test coupon, the test coupon comprising a partial length of a PDMstage, the test coupon including a rotor section received inside astator section, wherein the rotor section and the stator section areindependently rotatable about a longitudinal rotor axis and alongitudinal stator axis respectively, wherein further the rotor axisand the stator axis are substantially parallel and offset by apreselected axis offset distance, wherein further at least oneprogressing gap on a helical pathway is formed between the rotor sectionand the stator section when the rotor section and the stator section aredifferentially rotated; (b) receiving the test coupon inside a sealabletest chamber; (c) filling the test chamber with test fluid; (d) sealingthe test chamber; (e) rotating the rotor section, thereby actuatingcorresponding rotation of the stator section in the presence of testfluid in the progressing gap; (f) applying a braking torque to saidstator section rotation actuated in step (e); and (g) responsive to step(f), evaluating performance of the test coupon, wherein said evaluatingstep includes at least one substep selected from the group consistingof: (g1) controlling torque across the rotor section and the statorsection; (g2) counting, through to failure of the test coupon, at leastone of (1) rotor section rotation cycles and (2) stator section rotationcycles; (g3) examining at least one of (1) the rotor section and (2) thestator section for wear; and (g4) monitoring temperature change overtime in the test chamber. According further to the second aspect,embodiments of the test coupon, may be selected to be representative ofa full size PDM stage having a known rotor/stator eccentricity, in whichcase the preselected axis offset distance is selected to besubstantially the same as the known rotor/stator eccentricity.

Embodiments according to the first or second aspects may also includethe stator section providing an elastomer interface at a contact surfacewith the rotor section inside the test coupon, and in which theevaluating step (g) includes at least one substep selected from thegroup consisting of: (g1) controlling torque across the rotor sectionand the stator section; (g2) counting, through to failure of the testcoupon, at least one of (1) rotor section rotation cycles and (2) statorsection rotation cycles; (g3) examining at least one of (1) the rotorsection and (2) the stator section for wear; (g4) monitoring temperaturechange over time in the test chamber; and (g5) monitoring temperaturechange over time in the elastomer interface.

In other embodiments according to the first or second aspects, the atleast one progressing gap comprises a plurality of progressing gaps, andin which a first flow loop of test fluid is formed through the pluralityof progressing gaps when the test chamber is sealed and the rotorsection is rotated.

In other embodiments according to the first or second aspects, anannular cavity is formed between an external periphery of the statorsection and an internal periphery of the test chamber, wherein the atleast one progressing gap, a plurality of openings in the statorsection, and the annular cavity form a second flow loop of test fluidwhen the test chamber is sealed and the rotor section is rotated.

According to third and other aspects and embodiments, step (c) of themethod according to the first or second aspects requires no more thanabout 25 gallons of test fluid, and most preferably, no more than about2 gallons of test fluid. Embodiments of the test coupon may have anoverall longitudinal length of no more than about 12 inches. Embodimentsof the method according the first or second aspects may further comprisefurther steps of maintaining a preselected temperature and/or pressureinside the test chamber during at least steps (e) and (f). Embodimentsof the test coupon may further be cut from a full size PDM stage throughwhich drilling fluid was previously caused to flow. Step (f) of themethod according to the first or second aspects may be accomplishedusing a continuous-slip brake

It is therefore a technical advantage of the disclosed test apparatus tominiaturize the performance testing of PDM power sections in downholeconditions by accurately simulating such conditions (for example,temperature and stress loading) in a partial section of one stage of aPDM power section. The footprint of the disclosed test apparatus issuitable to house the apparatus in a conventional laboratory or even atthe wellsite.

A further technical advantage of the disclosed test apparatus is toprovide a PDM test apparatus that enables performance testing using asmall volume of drilling fluid. In this way, if desired, the disclosedtest apparatus can accurately simulate downhole service conditions usingsamples of actual drilling fluid extracted from a PDM previously indownhole service (such samples known to be available only in smallquantities).

A further technical advantage of the disclosed test apparatus is toavoid eccentric rotation vibration in the PDM power section test couponduring testing. By avoiding such vibration, the disclosed test apparatusenables performance evaluation of a PDM power section test coupon thatis isolated from the effects of vibration caused by the eccentricrotation of the rotor within the stator. Conventionally, theconstruction of PDM power sections provides a hard (e.g. metal) rotorsurface contacting a resilient stator (usually providing an elastomer orrubber through part or all of its cross-section). In normal PDM powersection operations, the eccentric rotation of the rotor within thestator imparts cyclic loads (including, without limitation, compressive,shear and tensile loads) on the resilient stator material, particularlyon the stator lobes. These loads imparted by cyclic contact are allincluded in the suite of performance metrics sought to be simulated andevaluated by the disclosed test apparatus.

However, the eccentric rotation of the rotor during normal PDM powersection operations also creates many modes of vibration throughout thePDM assembly and elsewhere on the drill string. The rotor on a full-sizedownhole PDM distributes torque and tilting reaction forces along thefull length of the elastomer component of the stator. The large lengthto diameter ratio of the power section allows these forces to bedistributed along the length of multiple power section stages. The PDM,over its entire multi-stage length, can therefore absorb the vigorousdynamic forces created as the rotor orbits eccentrically in the stator.In contrast, the disclosed test apparatus evaluates performance on onlya partial section of one PDM power section stage. It is therefore notpossible in the disclosed test apparatus to stabilize the rotor sectionwithin the test coupon against the vibrational effect of eccentricrotation and tilting forces as might be seen in “live” downholeoperations, since the length of the rotor section in the test coupon istoo short to have sufficient longitudinal flexure to compensate foreccentric rotation vibration.

Embodiments of the disclosed test apparatus therefore substantiallyeliminate eccentric rotation vibration from the test coupon by settingand holding the rotor rotation axis on a predetermined, fixed offsetfrom the stator rotation axis. The offset is selected to beapproximately equal to the design eccentricity of the full-size,operational power section whose performance the test coupon seeks toreplicate and evaluate. The disclosed test apparatus thus generates testresults that reflect performance isolated from substantially alleccentric rotation vibration, in a test coupon that can be very short inlength and requiring only a small volume of drilling fluid. In this way,the results generated by the disclosed test apparatus will moreaccurately predict corresponding or vibration-compensated orvibration-isolated performance expected in full-size PDM power sectionsoperating downhole.

The disclosed test apparatus provides yet a further advantage inembodiments in which eccentric rotation vibration is substantiallyeliminated. In embodiments where an offset is provided between axes ofrotor rotation and stator rotation (in order to elimination eccentricrotation vibration), the external drive train rotating the rotor isgreatly reduced in complexity which still giving a very accurate stressfield within the test coupon. In embodiments where an offset is notprovided between axes of rotor rotation and stator rotation, atransmission is required in the external drive train to counteract oreliminate eccentric rotation vibration. The rotor must also be longer insuch embodiments in order to generate flexure. Such an additionaltransmission and rotor length adds cost, complexity, and inevitably (1)increases potential footprint and (2) requires a higher volume ofdrilling fluid.

The foregoing has rather broadly outlined some features and technicaladvantages of the disclosed test apparatus, in order that the followingdetailed description may be better understood. Additional features andadvantages of the disclosed technology may be described. It should beappreciated by those skilled in the art that the conception and thespecific embodiments disclosed may be readily utilized as a basis formodifying or designing other structures for carrying out the sameinventive purposes of the disclosed technology, and that theseequivalent constructions do not depart from the spirit and scope of thetechnology as described.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of embodiments described in detailbelow, and the advantages thereof, reference is now made to thefollowing drawings, in which:

FIGS. 1-A through 1-J depict the operation of a conventional powersection 100 in a series of freeze-frame cutaway section views of theconventional power section in operation;

FIGS. 2-A through 2-J depict movement of corresponding components toFIGS. 1-A through 1-J in the disclosed new test apparatus 200, whereinsuch movement is also depicted in a series of freeze-frame cutawaysection views;

FIG. 3 shows an exemplary layout for test bed 300 on which componentsmay be secured for enabling the disclosed new test apparatus 200;

FIG. 4 illustrates test chamber 325 in cutaway view with internals andsurrounding components;

FIGS. 5 and 5A illustrate two different embodiments of test chamber 325in cutaway view, in which test fluid 323 may follow different flow loopsFL1 and FL2; and

FIG. 6 illustrates exemplary embodiments of drive train 310 as moregenerally illustrated on FIG. 3.

DETAILED DESCRIPTION

As discussed summarily above in this disclosure, this application isdirected to a PDM testing apparatus using a rotor/stator test couponthat is a partial length of one stage of a power section. A stage of apower section is defined by the minimum length to seal a single helicalprogressing cavity. The relationship describing the helical length of asingle progressing cavity is based on the rotor to stator lobe countratio and can be expressed as:Stator Pitch Length=(N+1)/N×Rotor Pitch LengthStator pitch length is a PDM design parameter chosen to give the desiredvolumetric fluid flow ratio for the selected rotor/stator configuration.

FIGS. 1-A through 1-J depict the mechanics of a conventional progressingcavity power section, as is well known in the prior art. Suchprogressing cavity power sections are also well known as “Moineau”devices. FIGS. 1-A through 1-J depict the operation of such aconventional power section 100 in a series of freeze-frame cutawaysection views of the power section in operation. The series offreeze-frame views depicted in FIGS. 1-A through 1-J are in sequence.Parts and other features of conventional power section 100 areidentified by reference number or letter, as described in detail furtherbelow. Where the same reference number or letter is used in FIGS. 1-Bthrough 1-J, the same part or feature of power section 100 is beingidentified on that Figure as depicted on FIG. 1-A. In this way,reference numbers and letters on FIGS. 1-B through 1-J can be omittedfor clarity on some views while still allowing the reader to understandthe subject matter depicted on FIGS. 1-B through 1-J.

As noted, power section 100 as depicted in FIGS. 1-A through 1-J isconventional. Such power sections are designed using intermeshingcontinuous helical pathways that provide, in cross section, cooperatinglobed gear profiles that intermesh with one another in an “n” and “n+1”combination. Typically the inner rotor has n lobes and the outer statorgear has n+1 lobes. Referring to FIG. 1A, rotor 120 is depicted with 6lobes, and stator 110 has 7 lobes, although these values are purely byway of example. As will be seen generally from FIGS. 1A through 1-Jviewed in sequence, the precise cross-sectional profiles of thesethese-gear-within-a-gear devices are normally described by hypocycloidgeometry created by rolling circle techniques. The resulting geardevices are helically swept over the axial lengths of the rotor andstator. The intermeshing along the axial length creates progressingcavities on a helical pathway between the rotor and stator, identifiedin cross-section on FIG. 1A as PC, through which a flow of fluid drivesrotor 120 around the inner periphery of stator 110.

The hypocycloid geometry of the intermeshed lobes on rotor 120 andstator 110 thus compels that in conventional PDM power sections such aspower section 100 on FIG. 1-A, rotor 120 orbits within stator 110 at aknown eccentricity from a central longitudinal axis. This eccentricityis a derived relationship and is related to the curvature and lobeheights generated from the mathematical expressions defining hypocycloidand true rolling motion used in the rotor and stator geometries. Reviewof FIGS. 1-A through 1-J in sequence illustrate this conventionaleccentricity in more detail.

Referring first to FIG. 1-A, stator 110 includes rotational marker 112and point marker 113. Rotational marker 112 sets a fixed point on theouter periphery of stator 110, and point marker 113 sets a fixed pointon the inner periphery of stator 110 (adjacent reference point “S1” onstator 110). It will be seen that the location of markers 112 and 113 onsubsequent views in FIGS. 1-B through 1-J does not change, indicatingthat stator 110 is stationary throughout the operation of conventionalpower section 100 depicted by FIGS. 1-A through 1-J.

With further reference to FIG. 1-A, rotor 120 includes fixed referencepoint “R1” on the outer periphery of one of its lobes. FIG. 1-A alsoshows progressing cavity (PC) position marker 130, on which PC markerline 132 indicates the position of the approximate most open point ofprogressing cavity PC. PC position marker 130 rotates in synch withprogressing cavity PC around a central longitudinal axis through stator110. PC position marker 130 thus also serves to illustrate and highlightthe eccentric rotation of rotor 120 within stator 110. It will be seenon FIG. 1-A that that PC position marker 130 is offset from a centrallongitudinal axis through rotor 120.

Referring now to FIG. 1-B, a flow of drilling fluid (not illustrated)has displaced rotor 120 within stator 110 such that progressing cavityPC has rotated approximately 90 degrees clockwise within stator 110 fromthe corresponding progressing cavity position depicted on FIG. 1-A. Thisrotation of progressing cavity PC is illustrated by the new position ofPC marker line 132 on FIG. 1-B, as well as a new general position onFIG. 1-B of progressing cavity PC. FIG. 1-B also shows that in responseto clockwise movement of PC, rotor 120 has rotated counterclockwise asshown by the new position of rotor reference point R1. FIG. 1-Billustrates the clockwise movement of progressing cavity PC by arrowR_(PC), and the responsive counterclockwise movement of rotor 120 byarrow R_(R).

FIG. 1-C illustrates that the flow of drilling fluid has displaced rotor120 within stator 110 such that progressing cavity PC has rotatedapproximately a further 90 degrees clockwise within stator 110 from thecorresponding progressing cavity position depicted on FIG. 1-B. Therelative movement of components within stator 110 can be seen from thenew positions of progressing cavity PC, PC marker line 132, and rotorreference point R1 on FIG. 1-C, as compared to their correspondingpositions on FIGS. 1-A and 1-B.

FIGS. 1-D and 1-E each show a further rotation of approximately 90degrees clockwise of progressing cavity PC over the previouslyillustrated positions. Again, the relative movement of components withinstator 110 can be seen from the new positions of progressing cavity PC,PC marker line 132, and rotor reference point R1 on each advancing viewon FIGS. 1-A through 1-E. It will be appreciated that on FIG. 1-E,progressing cavity PC has made one complete revolution of stator 110over the starting position on FIG. 1-A.

FIGS. 1-F through 1-J illustrate the counterclockwise rotation of rotor120 within stator 110 responsive to subsequent full revolutionsclockwise of progressing cavity PC. In each of FIGS. 1-A, and then 1-Fthrough 1-J, progressing cavity PC has made one further full clockwiserevolution over the previously illustrated view. When FIGS. 1-A, andthen 1-F through 1-J are viewed in sequence, the new position of rotorreference point R1 can be seen in response to one additional revolutionof progressing cavity PC. It will be appreciated that in FIG. 1-J,progressing cavity PC has made 6 clockwise revolutions of stator 110over the view depicted in FIG. 1-A, during which time rotor referencepoint R1 has made one counterclockwise revolution, indicating that theeccentricity of rotor 120 within stator 110 is in the same state in FIG.1-J as it was initially in FIG. 1-A.

As noted throughout the disclosure immediately above, FIGS. 1-A through1-J illustrate movement of components within a conventional powersection 100. FIGS. 2-A through 2-J depict movement of correspondingcomponents within the disclosed new test apparatus 200. As with FIGS.1-A through 1-J, FIGS. 2-A through 2-J depict such movement within testapparatus 200 in a series of freeze-frame cutaway section views of thetest apparatus in operation. The series of freeze-frame views depictedin FIGS. 2-A through 2-J are in sequence. Similar to conventional powersection 100 depicted in FIGS. 1-A through 1-J, test apparatus 200 inFIGS. 2-A through 2-J provides a rotor 220 with 6 lobes operating insidea stator 210 with 7 lobes. Where the same reference number or letter isused in FIGS. 2-A through 2-J, the same part or feature of testapparatus 200 is being identified. In this way, reference numbers andletters on FIGS. 2-A through 2-J can be omitted for clarity on someviews while still allowing the reader to understand the subject matterdepicted on FIGS. 2-A through 2-J.

The “Summary” section describes above how the disclosed test apparatus200 tests less than one full length of a full downhole PDM power sectionstage, and thus necessarily cannot provide a series of full progressingcavities (one full stage of a power section being defined by the minimumlength to seal a single helical progressing cavity, see above). Testapparatus 200 thus cannot be sealed to operate conventionally withmoving drilling fluid driving a rotor in a stationary stator (asillustrated and described above with reference to FIGS. 1-A through1-J). Test apparatus 200 thus does not provide progressing cavities asillustrated as PC on FIGS. 1-A through 1-J. In contrast, as illustratedon FIGS. 2-A though 2-J, test apparatus 200 provides progressing gaps PGon helical pathways formed between rotor 220 and stator 210 when rotor220 and stator 210 are differentially rotated. Progressing gaps PG onFIGS. 2-A though 2-J are necessarily only a partial section ofcorresponding progressing cavities PC found on a full PDM power stage,as illustrated on FIGS. 1-A through 1-J.

In operation, currently preferred embodiments of test apparatus 200 onFIGS. 2-A through 2-J provide an external motor (not illustrated onFIGS. 2-A through 2-J) to rotate rotor 220 while submersed in testfluid/drilling fluid inside stator 210. As rotor 220 rotates, itactuates corresponding rotation of stator 210 via contact between rotor220 and stator 210. An external brake mechanism (again not illustratedon FIGS. 2-A through 2-J) is then activated to apply braking torque tointentionally slow the rotation of stator 210 against the poweredrotation of rotor 220. Applied braking torque can be finely controlled.In this way, a controlled stress field can be intentionally introducedon the components inside test apparatus 200 that approximate closely inkind, location, and strength the operational stresses experienced insideconventional power section 100 on FIGS. 1-A through 1-J when drillingfluid is pumped through progressing cavity PC to rotate rotor 120 withinstationary stator 110.

Looking at FIGS. 2-A through 2-J in more detail, FIG. 2-A illustratestest apparatus 200 with the following parts and features, many of whichare counterparts to the corresponding parts and features described abovewith respect to FIGS. 1-A through 1-J:

Stator 210

Stator rotational marker 212

Stator point marker 213

Stator reference point S1

Rotor 220

Rotor reference point R1

Progressing gap PG along the axial length of rotor 220 and stator 210,on a helical pathway formed between rotor 220 and stator 210 when rotor220 and stator 210 are differentially rotated.

Additionally, FIG. 2-A illustrates rotor rotation marker line 222,which, along with rotor reference point R1, indicates rotation of rotor220 relative to other components in test apparatus 200 as views advancethrough FIGS. 2-A through 2-J.

Referring now to FIG. 2-B, external rotor motor (not illustrated) hasrotated rotor 220 approximately 90 degrees counterclockwise withinstator 210 from the corresponding position of rotor 220 depicted on FIG.2-A. As a result, rotational motion forces from rotor 220 have exertedthemselves on stator 210, actuating rotation of stator 210 in acounterclockwise direction. This displacement of stator 210 can be seenby comparing the relative positions of stator rotational marker 212,stator point marker 213 and stator reference point S1 on FIG. 2-B ascompared to FIG. 2-A. Additionally the displacement of rotor 220 byapproximately 90 degrees counterclockwise in FIG. 2-B has causedprogressing gap PG to move in a clockwise direction away from itscorresponding position in FIG. 2-A. This clockwise movement ofprogressing gap PG is best seen by comparing its displaced positionrelative to stator rotational marker 212 in FIG. 2-A and then FIG. 2-B.FIG. 2-B illustrates the counterclockwise movement of rotor 220 by arrowR_(R), the responsive counterclockwise movement of stator 210 by arrowR_(S), and the resulting clockwise movement of progressing gap PG byarrow R_(PG).

FIG. 2-C illustrates that external rotor motor (not illustrated) hasrotated rotor 220 approximately 90 degrees further counterclockwisewithin stator 210 from the corresponding position of rotor 220 depictedon FIG. 2-B. As a result, rotational motion forces from rotor 220 haveacted on stator 210, causing stator 210 to rotate further in acounterclockwise direction. This displacement of stator 210 can be seenby comparing the relative positions of stator rotational marker 212,stator point marker 213 and stator reference point S1 on FIG. 2-C ascompared to FIG. 2-B. Additionally the displacement of rotor 220 byapproximately 90 degrees further counterclockwise in FIG. 2-C has causedprogressing gap PG to move in a clockwise direction away from itscorresponding position in FIG. 2-B, as seen by comparing the position ofprogressing gap PG in FIG. 2-C relative to stator rotational marker 212in FIG. 2-B.

FIGS. 2-D and 2-E each show a further rotation of approximately 90degrees counterclockwise of rotor 220 over the previously illustratedpositions. Again, the relative movement of components within stator 210can be seen from the new positions of rotor rotation marker line 222,rotor reference point R1, stator rotational marker 212, stator pointmarker 213, stator reference point S1, and progressing gap PG on eachadvancing view on FIGS. 2-A through 2-E. It will be appreciated that onFIG. 2-E, external rotor motor (not illustrated) has caused rotor 220 tomake one complete revolution of stator 210 over the starting position onFIG. 2-A.

FIGS. 2-F through 2-J illustrate the counterclockwise rotation of stator210 responsive to subsequent full revolutions counterclockwise of rotor220. In each of FIGS. 2-A, and then 2-F through 2-J, rotor 220 has madeone further full counterclockwise revolution over the previouslyillustrated view. When FIGS. 2-A, and then 2-F through 2-J are viewed insequence, the new relative position of stator rotational marker 212,stator point marker 213, stator reference point S1, and progressing gapPG can be seen in response to one additional revolution of rotor 220. Itwill be appreciated that in FIG. 2-J, rotor reference point R1 has made6 counterclockwise revolutions as compared to the view depicted in FIG.2-A, during which time stator reference point S1 has made almost onecounterclockwise revolution in the orbital distance separating rotorreference point R1 and stator reference point R1 during rotation. Itwill be appreciated that with one further complete counterclockwiserevolution of rotor reference point R1 (a seventh overall rotation),stator reference point S1 will have made one complete counterclockwiserevolution in the orbital distance separating rotor reference point R1and stator reference point S1, and will further have returned to thesame position relative to rotor reference point R1 as depicted in FIG.2-A.

Returning now to view FIGS. 1-A through 1-E in sequence, it will beappreciated rotor 120 is in an eccentric orbit within stator 110 duringin the operation of conventional power section 100. Conventionally, theconstruction of PDM power sections provides a hard (e.g. metal) rotorsurface contacting a resilient stator (usually providing an elastomer orrubber through part or all of its cross-section). As described above inthe “Summary” section, in normal PDM power section operations, theeccentric rotation of the rotor within the stator imparts cyclic loads(including, without limitation, compressive, shear and tensile loads) onthe resilient stator material, particularly on the stator lobes. Theeccentric rotation of the rotor during normal PDM power sectionoperations also creates many modes of vibration throughout the PDMassembly and elsewhere on the drill string. It will be appreciated thatthese vibrations, as experienced downhole, can be addressed over thelength of a full power section stage by fixing the ends of the rotor andallowing the natural flexure of the rotor to compensate. However, inshort lengths of conventional power section configured according toFIGS. 1-A through 1-E, such eccentric rotation vibrations cannot easilybe compensated for, since the rotor is too short to have the requiredflexure.

Turning now to view FIGS. 2-A through 2-E in sequence, it will beappreciated that embodiments of the disclosed test apparatus 200 arereconfigured in a way to optimize, and in some cases to substantiallyeliminate, any eccentric rotation vibration that might potentially arisein the test coupon, so that performance evaluation of the components ofthe test coupon can be conducted free of the effects of such vibration.FIGS. 2-A through 2-E show that stator 210 and rotor 220 each rotateindependently about their own fixed, substantially parallel longitudinalaxes, and that the rotor rotation axis is set at a fixed offset distancefrom the stator rotation axis. The fixed offset distance is apredetermined design choice, selected according to the size and lobecount of the rotor and stator, and further according to the amount ofoperational contact desired by the rotor on the stator as the rotor isexternally rotated. Comparison should now be made with the eccentricorbit of rotor 120 within stator 110 on FIGS. 1-A through 1-E insequence, where rotor 120 makes operational contact on stator 110 asrotor 120 moves around the inner periphery of stator 110. Returning toFIGS. 2-A through 2-E, it will be appreciated that by selecting, settingand holding the rotor rotation axis of rotor 220 at a fixed offsetdistance from the rotation axis of stator 210, the same operationalcontact between rotor 220 and stator 210 can be replicated in testapparatus 200 as is experienced in conventional PDM power section 100 onFIGS. 1-A through 1-E. In test apparatus 200 on FIGS. 2-A through 2-E,however, there is no eccentric orbital rotation of rotor 220 withinstator 210, thereby substantially eliminating vibration that mightotherwise be caused by such eccentric orbital rotation.

Previous disclosure described how in most test environments, testapparatus 200 will be configured such that the offset between of rotor220 and stator 210 is substantially the same as the eccentricity of therotor's orbit in a full-size PDM whose performance the test coupon isdesigned to evaluate. For purposes of this paragraph, such offsetbetween longitudinal rotational axes of rotor 210 and stator 220 will bereferred to as the “ideal eccentricity”. Additional embodiments of testapparatus 200 may be configured with variations in offset (greater orsmaller) away from the ideal eccentricity. Such additional embodimentswill simulate (and enable corresponding performance evaluation under)extreme loading conditions experienced by full-length PDM power sectionsin environments where the ambient dynamic loading conditions aredeflecting rotor's eccentric orbit beyond design.

Physical embodiments of the disclosed test apparatus are now describedwith reference to FIGS. 3 through 6. As before, where the same referencenumber or letter is used in FIGS. 3 through 6, the same part or featureis being identified on more than one Figure.

FIG. 3 shows an exemplary layout for test bed 300 on which componentsmay be secured for enabling the disclosed test apparatus. External motor305 provides rotational power to rotor section 315 via drive train 310.External motor 305 is illustrated on FIG. 3 as an electric motor. Thescope of the disclosed test apparatus is indifferent, however, to thetype of external motor selected. Drive train 310 is illustrated in FIG.3 as a belt-and-pulley drive train. Embodiments of test bed 300 thatprovide drive train 310 as a belt-and-pulley train will gain furtheradvantages as described below with reference to FIG. 6. However, thisdisclosure is not limited to embodiments whose drive train 300 is abelt-and pulley train.

Rotor section 315 on FIG. 3 is set rotationally in place on rotorbearings 318. It will be understood from momentary reference to FIG. 5that a distal end of rotor section 315 terminates inside test chamber325, and functions as the rotor portion of the test coupon underanalysis in test chamber 325. Test chamber 325 is shown sealed on FIG.3. Embodiments of the internals of test chamber 325 are discussedfurther below with reference to FIGS. 4, 5 and 5A. Returning to FIG. 3,stator shaft 320 exits test chamber 325 and will be understood to berotationally connected to stator section 321 inside test chamber 325(again, refer momentarily to FIG. 4). Stator bearings 328 set statorshaft 320 rotationally in place. Stator shaft 320 is rotationallyconnected to torque sensor 330, which measures the torque generated bystator shaft 320 as stator shaft 320 rotates in response to externalmotor 305 driving rotor 315. Torque sensor 330 is further subject tointentional slowing of rotation via a braking torque supplied bycontinuous-slip brake 335, whereby fine control of the braking torqueinduces a controlled stress field (not illustrated) between rotor 315and stator 321 inside test chamber test chamber 325. While theembodiments described with reference to FIG. 3 refer to acontinuous-slip brake 335, it will be understood that the scope of thetest apparatus is not limited in this regard, and that other types ofconventional brakes may be substituted for continuous-slip brake 335.

Test chamber 325 will now be discussed in more detail with reference toFIGS. 4, 5 and 5A. FIGS. 4, 5 and 5A are section views shown generallyon FIG. 3. Note FIG. 3 omits specific reference to FIG. 5A mainly forclarity, it being understood from disclosure further below that FIGS. 5and 5A are alternative embodiments and therefore the section line forFIG. 5 shown on FIG. 3 is representative for both FIGS. 5 and 5A.

Referring first to FIG. 4, test chamber 325 is seen in cutaway view.Rotor section 315 is omitted for clarity. Stator section 321 is seenrotationally and coaxially fixed to stator shaft 320. Sealable testchamber 325 also provides annular cavity 322 formed between an externalperiphery of stator section 321 and an internal periphery of testchamber 325. Per the embodiment of FIG. 5A, described further below,FIG. 4 depicts openings 324 provided stator section 321. (It will beappreciated from disclosure further below that openings 324 are notprovided in the embodiment of FIG. 5). In operation, test chamber 325 isfilled with test fluid 323 (advantageously drilling fluid) prior tobeing sealed so that stator section 321 is completely immersed.

FIGS. 5 and 5A illustrate two exemplary alternative embodiments of testchamber 325 in more detail with surrounding components, again in cutawayview. FIGS. 5 and 5A each depict progressing gaps PG formed on helicalpathways between stator section 321 and rotor section 315. Test chamber325 is sealed and filled with test fluid 323. It was noted in earlierdisclosure that test fluid 323 is present in test chamber 325 tointeract with the materials from which rotor section 315 and statorsection 321 are made, in order to simulate degradation that might beseen in actual downhole conditions. It was also noted in earlierdisclosure, however, that once test chamber 325 was sealed, the relativedifferential rotation of rotor section 315 and stator section 321 maycause the test fluid 323 to flow under low pressure through progressinggaps PG. Referring to the embodiment of FIG. 5, and as illustrated bythe darker arrows on FIG. 5, first flow loop FL1 for test fluid 323 isformed through progressing gaps PG. FIG. 5 depicts first flow loop FL1in one exemplary flow direction, although it will be understood thatflow loop FL1 may arise in either direction according to user selectionof the direction in which to rotate rotor section 315.

Referring now to the embodiment of FIG. 5A, openings 324 are provided instator section 321. The darker arrows on FIG. 5A illustrate that secondflow loop FL2 for test fluid 323 may form in one exemplary flowdirection, through progressing gaps PG in a direction away from the endnear rotor bearing 318, then through openings 324 in stator section 321,then through annular cavity 322 back to the rotor bearing end ofprogressing gaps PG. Again, although not specifically illustrated onFIG. 5A, it will be understood that, according to user selection of thedirection in which to rotate rotor section 315, second flow loop FL2 mayflow in either direction. In general, first and second flow loops FL1and FL2 on FIGS. 5 and 5A are indifferent to the direction in which testfluid 323 may be caused to flow.

FIGS. 5 and 5A further show rotor section 315 held rotationally in placeby one of rotor bearings 318. Stator shaft 320 is also shown heldrotationally in place by one of stator bearings 328. FIG. 5 furtherillustrates a cross-section cut for FIGS. 2-A through 2-J. It will beunderstood that freeze-frame views seen on FIGS. 2-A through 2-Jrepresent movement of the disclosed test apparatus within test chamber325 along an exemplary cross-section cut line as shown on FIG. 5.

FIGS. 5 and 5A also illustrate the preselected offset 317 of rotorrotation axis 316 and stator rotation axis 326. As discussed extensivelyabove with reference to FIGS. 2-A through 2-E, for example, offset 317is introduced to eliminate eccentric rotation vibration during operationof test chamber 325. It will be understood that relative adjustment ofrotor bearings 318 and stator bearings 328 allows test chamber 325 toaccommodate a range of offsets 317 to be selected, set and heldaccording to user requirements.

FIG. 6 illustrates exemplary embodiments of drive train 310 as moregenerally illustrated on FIG. 3. On FIG. 6, three alternatives areillustrated in which varying drive speeds and torques may be deliveredto rotor section 315 by external motor 305 via drive train 310. It willbe understood, however, that the disclosed test apparatus is not limitedto the three alternatives illustrated on FIG. 6, and that the scope ofthe disclosed test apparatus contemplates many alternative drive speedsand torques delivered to rotor section 315. It will be furtherappreciated that by providing different drive speeds and torques torotor section 315, the disclosed-test apparatus can simulate themutation speed of a PDM power section, i.e. the “step down” effect ofthe “gear within the gear”.

As suggested in earlier disclosure, external motor 305, drive train 310,rotor and stator bearings 318 and 328, torque sensor 330,continuous-slip brake 335, and other seals not called out by part numberare all off-the-shelf components whose performance characteristics areselected to suit a particular design of the disclosed test apparatus. Inpresently preferred embodiments, a suitable external motor 305 is aBrook Crompton 75 HP AC 3-phase 230/460 V motor delivering up to 1800rpm; a suitable continuous-slip brake 335 is a Wichita Clutch model KKB208; suitable seals for test chamber 325 are UTEX models MP; suitablerotor/stator bearings 318/328 are available from Dodge; and a suitabletorque transmitter 330 is a Himmelstein HRCT 39000X. The scope of thedisclosed test apparatus is nonetheless not limited to any particularselection or combination of such off-the-shelf components. Likewise, thecontrol of the disclosed test apparatus is advantageously viaconventional PLC and PID control, and the scope of the disclosed testapparatus is not limited in this regard.

Referring now to commonly-invented and commonly-assigned U.S.Provisional Patent Application Ser. No. 62/311,278 (the “ProvisionalApplication”), to which this disclosure claims priority and whoseprovisional disclosure is incorporated herein by reference, FIG. 7 ofthe Provisional Application is an exemplary finite element analysis(FEA) image of static displacement (strain) based upon known torquestresses placed on a rotor to be used in conjunction with the disclosedtest apparatus. The image is color coded to show increasing strain. FEAimages of the type shown on FIG. 7 of the Provisional Application areuseful, for example, for sizing the external motor driving the rotor andderiving specifications for the interconnecting drive train.

There now follows description of an exemplary operation of the disclosedtest apparatus. It will be understood that the following disclosure isfor illustration only, and that the disclosed test apparatus is notlimited thereby.

The objectives of an exemplary test protocol may include to evaluate newelastomer compounds in an environment that accurately simulates expecteddownhole service in an operational PDM power section. With this in mind,a test stator section is prepared with the elastomer, molded into theactual stator profile to be expected downhole. The test stator sectionis placed into the test chamber. In accordance with the disclosed testapparatus, the stator section is then (1) exposed the actual drillingfluids expected downhole; (2) exposed to the actual elevatedtemperatures expected downhole; (3) loaded with comparative (or higher)forces and cycle frequencies expected downhole. The disclosed testapparatus may then, for example, measure the number of cycles to failureunder defined loads and conditions. In some embodiments, the cycles tofailure may be determined by counting, through to failure, the rotorsection cycles and/or the stator section cycles. In other embodiments,the rotor section and/or stator section may be examined for wear. Inother embodiments, temperature change over time inside the test chambermay be monitored. In other embodiments, temperature change over time maybe monitored in an elastomer layer provided by the stator section at itscontact surface with the rotor section. The test data yielded by thedisclosed test apparatus will be expected to correlate closely tocomparative test data that might have been extracted from a hypotheticalpower section in downhole service, and may be used to develop elastomercompounds with improved performance characteristics according to theservice. Alternatively, without limitation, the disclosed test apparatusmay be used to test the performance of actual test coupons andsurrounding drilling fluids taken from PDMs in service in wells withtheir own chemistry.

Embodiments of the disclosed test apparatus may be expected to achievethe following exemplary target performance parameters (again, thefollowing list is not exhaustive, and the scope of the disclosed testapparatus is not limited in any of the following regards):

Test chamber temperatures up to 350-400 degrees F.;

Ability to use stator sections or rotor sections cut from actualdownhole tools; and

Ability to load stator elastomers up to approximately 40% strain.

It will be appreciated that the scope of the disclosed test apparatus isnot limited to the construction of stator sections that may be put inthe test coupon, and includes, without limitation, all-elastomerconstruction, hybrid metal/elastomer constructions (“evenwall”) or othertypes of construction. Likewise, the disclosed test apparatus is notlimited to the size, type or construction of rotor that may be put inthe test coupon.

Alternative embodiments of the disclosed test apparatus could furtherinclude, without limitation, the following features and aspects:

(a) Substituting the disclosed offset shaft mounting of rotor and statorand convert the rotor to an eccentric transmission by which to receiverotational torque;

(b) Adapting the disclosed test apparatus to evaluate miniaturized 1.0to 2.0 stage motors, advantageously with small diameters; and

(c) Adapting the disclosed test apparatus to evaluate test coupons withstraight pathways for drilling fluid rather than helical pathways.

(d) Adapting the input motor to deliver impulse loads to the rotor orstator shaft to simulate downhole torsional impacts and stall events.

(e) Varying the number and locations of rotor/stator bearings forsupport of the rotor/stator.

(f) Varying the number and locations of the test chamber fluid seals.

Although the inventive material in this disclosure has been described indetail along with some of its technical advantages, it will beunderstood that various changes, substitutions and alternations may bemade to the detailed embodiments without departing from the broaderspirit and scope of such inventive material as set forth in thefollowing claims.

We claim:
 1. A method for measuring load performance of a rotor/statortest coupon, the method comprising the steps of: (a) providing a testcoupon, the test coupon including a rotor section received inside astator section; (b) rotating the rotor section, wherein rotation of therotor section actuates corresponding rotation of the stator section; (c)applying a braking torque to said stator section rotation actuated instep (b); and (d) responsive to step (c), evaluating load performance ofthe test coupon.
 2. The method of claim 1, in which step (d) includes atleast one substep selected from the group consisting of: (d1)controlling torque across the rotor section and the stator section; (d2)counting, through to failure of the test coupon, at least one of (1)rotor section rotation cycles and (2) stator section rotation cycles;and (d3) examining at least one of (1) the rotor section and (2) thestator section for wear.
 3. The method of claim 1, in which the testcoupon comprises at least a partial length of a positive displacementmotor (PDM) stage.
 4. The method of claim 1, in which, in step (a), atleast one progressing gap on a helical pathway is formed between therotor section and the stator section when the rotor section and thestator section are differentially rotated.
 5. The method of claim 1,further comprising, after step (a) and before step (b), the substeps of:(a1) receiving the test coupon inside a sealable test chamber; (a2)filling the test chamber with test fluid; and (a3) sealing the testchamber.
 6. The method of claim 5, in which step (d) further includesthe substep of monitoring temperature change over time in the testchamber.
 7. The method of claim 1, in which the stator section providesan elastomer interface at a contact surface with the rotor sectioninside the test coupon, and in which step step (d) includes at least onesubstep selected from the group consisting of: (d1) controlling torqueacross the rotor section and the stator section; (d2) counting, throughto failure of the test coupon, at least one of (1) rotor sectionrotation cycles and (2) stator section rotation cycles; (d3) examiningat least one of (1) the rotor section and (2) the stator section forwear; and (d4) monitoring temperature change over time in the elastomerinterface.
 8. The method of claim 5, in which the stator sectionprovides an elastomer interface at a contact surface with the rotorsection inside the test coupon, and in which step (d) further includesthe substep of monitoring temperature change over time in the testchamber.
 9. The method of claim 5, in which: (1) in step (a), at leastone progressing gap on a helical pathway is formed between the rotorsection and the stator section when the rotor section and the statorsection are differentially rotated; and (2) a first flow loop of testfluid is formed through the at least one progressing gap when the testchamber is sealed and the rotor section is rotated.
 10. The method ofclaim 5, in which: (1) in step (a), at least one progressing gap on ahelical pathway is formed between the rotor section and the statorsection when the rotor section and the stator section are differentiallyrotated; and (2) an annular cavity is formed between an externalperiphery of the stator section and an internal periphery of the testchamber, wherein the at least one progressing gap, a plurality ofopenings in the stator section, and the annular cavity form a secondflow loop of test fluid when the test chamber is sealed and the rotorsection is rotated.
 11. The method of claim 1, in which the rotorsection and the stator section are rotated about a longitudinal rotoraxis and a longitudinal stator axis respectively, and in which the rotoraxis and the stator axis are offset by a preselected axis offsetdistance.
 12. The method of claim 11, in which the test coupon isselected to be representative of a full size PDM stage having a knownrotor/stator eccentricity, and in which the preselected axis offsetdistance is selected to be substantially the same as the knownrotor/stator eccentricity.
 13. The method of claim 1, in which the testcoupon is cut from a full size PDM stage through which drilling fluidwas previously caused to flow.
 14. The method of claim 5, in which thetest fluid was previously passed through a full size PDM stage.
 15. Amethod for measuring load performance of a rotor/stator test coupon, themethod comprising the steps of: (a) providing a test coupon, the testcoupon including a rotor section received inside a stator section; (b)rotating the stator section, wherein rotation of the stator sectionactuates corresponding rotation of the rotor section; (c) applying abraking torque to said rotor section rotation actuated in step (b); and(d) responsive to step (c), evaluating load performance of the testcoupon.
 16. The method of claim 15, in which step (d) includes at leastone substep selected from the group consisting of: (d1) controllingtorque across the rotor section and the stator section; (d2) counting,through to failure of the test coupon, at least one of (1) rotor sectionrotation cycles and (2) stator section rotation cycles; and (d3)examining at least one of (1) the rotor section and (2) the statorsection for wear.
 17. The method of claim 15, in which the test couponcomprises at least a partial length of a positive displacement motor(PDM) stage.
 18. The method of claim 15, in which, in step (a), at leastone progressing gap on a helical pathway is formed between the rotorsection and the stator section when the rotor section and the statorsection are differentially rotated.
 19. The method of claim 15, furthercomprising, after step (a) and before step (b), the substeps of: (a1)receiving the test coupon inside a sealable test chamber; (a2) fillingthe test chamber with test fluid; and (a3) sealing the test chamber. 20.The method of claim 19, in which step (d) further includes the substepof monitoring temperature change over time in the test chamber.
 21. Themethod of claim 15, in which the stator section provides an elastomerinterface at a contact surface with the rotor section inside the testcoupon, and in which step step (d) includes at least one substepselected from the group consisting of: (d1) controlling torque acrossthe rotor section and the stator section; (d2) counting, through tofailure of the test coupon, at least one of (1) rotor section rotationcycles and (2) stator section rotation cycles; (d3) examining at leastone of (1) the rotor section and (2) the stator section for wear; and(d4) monitoring temperature change over time in the elastomer interface.22. The method of claim 19, in which the stator section provides anelastomer interface at a contact surface with the rotor section insidethe test coupon, and in which step (d) further includes the substep ofmonitoring temperature change over time in the test chamber.
 23. Themethod of claim 19, in which: (1) in step (a), at least one progressinggap on a helical pathway is formed between the rotor section and thestator section when the rotor section and the stator section aredifferentially rotated; and (2) a first flow loop of test fluid isformed through the at least one progressing gap when the test chamber issealed and the rotor section is rotated.
 24. The method of claim 19, inwhich: (1) in step (a), at least one progressing gap on a helicalpathway is formed between the rotor section and the stator section whenthe rotor section and the stator section are differentially rotated; and(2) an annular cavity is formed between an external periphery of thestator section and an internal periphery of the test chamber, whereinthe at least one progressing gap, a plurality of openings in the statorsection, and the annular cavity form a second flow loop of test fluidwhen the test chamber is sealed and the rotor section is rotated. 25.The method of claim 15, in which the rotor section and the statorsection are rotated about a longitudinal rotor axis and a longitudinalstator axis respectively, and in which the rotor axis and the statoraxis are offset by a preselected axis offset distance.
 26. The method ofclaim 25, in which the test coupon is selected to be representative of afull size PDM stage having a known rotor/stator eccentricity, and inwhich the preselected axis offset distance is selected to besubstantially the same as the known rotor/stator eccentricity.
 27. Themethod of claim 15, in which the test coupon is cut from a full size PDMstage through which drilling fluid was previously caused to flow. 28.The method of claim 19, in which the test fluid was previously passedthrough a full size PDM stage.